Updated

ISIS used “sharp tools” to murder 33 people in Syria on Wednesday, subsequently dumping the bodies into a mass grave “filled with blood,” a human rights group said.

The Syrian Observatory for Human Rights said the slaughter was “the largest execution operation carried out by [ISIS] in 2017.”

The identities of the dead – men aged 18-25 – were not immediately known, and it was unclear if they were civilians or captured soldiers. “Marks of slaughter” appeared on the victims’ necks.

The killings occurred near the town of Mayadeen in Deir el-Zour province, close to the Syrian border with Iraq. Opposition activist Omar Abu Laila, who is from Deir el-Zour but currently lives in Europe, also reported the murders.

ISIS has carried out similar killings in Syria this year. When it controlled the ancient Syrian city of Palmyra, ISIS routinely carried out public beheadings, including of the antiquities chief, whose body was hung from a pole in a main square.

The Associated Press contributed to this report.